The present invention relates to an evaporable getter device that can be activated in less time.
It is known to use getter materials to help to maintain a vacuum for a long period of time. Kinescopes, including both conventional cathode-ray types and flat panel displays, use getter materials to fix trace gas that remain after initial evacuation or that result from out gassing of the materials used to make the kinescope.
The getter material most commonly used in kinescopes is metallic barium that is applied as a thin film on an inner wall of the kinescope. Devices known as evaporable getters apply the barium film after the kinescope has been evacuated and hermetically sealed. These getter devices comprise an open metal container that contains a compound of barium and aluminium, BaAl.sub.4, in powder form, and nickel, Ni, in a powder form, in about equal ratios by weight. These types of getter devices are well known in the art as exemplified by U.S. Pat. No. 5,118,988, which is assigned to the assignee of this application.
Once evacuated and sealed, a coil located outside the kinescope induction-heats the getter device. This heating activates the getter device by causing the barium to evaporate. The metal in the container heats most rapidly and transfers heat to the powders that it contains.
The following reaction takes place when the temperature in the powders reaches about 800.degree. C.: EQU BaAl.sub.4 +4Ni.fwdarw.Ba+4NiAl (I)
This reaction is strongly exothermic and heats the powders to a temperature of about 1200.degree. C. The barium evaporates at this temperature and then sublimates on the walls of the kinescope to form a metallic film.
To obtain a good reactivity in the powder packet, the BaAl.sub.4 compound is in a powder form in which the particles have a size that is smaller than about 250 .mu.m. The nickel is also in a powder form and usually has a particle size that is smaller than 30 .mu.m, although small amounts of the powder can have a somewhat larger particle size up to about 50 .mu.m.
The morphology of the nickel powder differs among different manufacturers of getter devices. The same manufacturer may use types of nickel having different morphologies for different getter devices. However, no commercially available getter device is known to contain nickel particles having two or more different morphologies. The most common morphology for the nickel, as shown in FIG. 1, is one in which the nickel particles are essentially spherical in as much as the particles have a generally rounded shape with a relatively smooth surface. Another type of particle uses particles having a dendritic morphology as shown in FIG. 2 in which the shape of the particles is less regular and the surface of the particles is relatively nutated.
One way to characterize the morphologies of particles is by "specific area" which is the surface area per unit weight. This aspect of particle size can be measured using instruments that implement the Brunauer-Emmett-Teller (B.E.T.) theory. B.E.T. instruments, well known in the art of measuring and characterizing powders, provide a way to measure the surface area from absorbed gasses (e.g. N.sub.2 at low temperature) as a function of pressure. Known nickel powders having a rounded or spherical morphology have B.E.T. specific areas in the ranges of 0.25-0.35 m.sup.2 /gram, whereas known dendritic particles have B.E.T specific areas in the ranges of 0.38-0.50 m.sup.2 /gram.
The amount of time needed to evaporate a predetermined amount of barium from the getter device, measured from the time when energy is first supplied to the device from the coil, is usually defined in the art as "Total Time". The phrase "Total Time", and its shortened form "TT", will be used in the following specification.
To function properly, modern color kinescopes may require a film containing as much as about 300 mg of barium. The current TT for evaporating such an amount of barium is about 40 seconds, or about 7.5 mg per second. The delay represented by the total time TT is a "bottle-neck" in the production of modern kinescopes. Therefore, there is a need in the art to have getter devices that can evaporate the same amount of barium in a shorter TT than can current getter devices.
One way to try to decrease the TT is to try increasing the power supplied by the coil. Another way is to try increasing the reactivity of the powders by decreasing their particle size. However, for the reasons explained below, neither of these solutions is practical.
Increasing the coil power with the current getter devices does not work because the powder container heats so quickly that there is not enough time to transmit the heat through the packet of powders. This rapid heating at the container causes the temperature of the powders directly against the container to become much higher than the temperature in the rest of the packet of powders. The reaction between BaAl.sub.4 and Ni begins in the powders against the container, and the barium vapors produced in this area of the powders packet produces a vapor pressure having enough force to expel reaction fragments (principally BaAl.sub.4, Ni and NiAl). This must be prevented so as not to compromise the working of the kinescope, and also has the potential to reduce the amount of barium being evaporated.
Decreasing the particle size of the powder can also cause an excessive, localized increase in the rate of the reaction between BaAl.sub.4 and Ni next to the container and may also eject reaction fragments.
It is an objective of the present invention to provide an evaporable getter device that can be activated in less time without the disadvantages encountered in other systems. Specifically, it is an objective of the present invention to smoothly evaporate barium, without ejecting reaction fragments, more rapidly than previously possible using known evaporable getter devices.
The objectives of the present invention can be achieved according to the present invention using an evaporable getter device that comprises a metal container that contains the BaAl.sub.4 powder and nickel powder in which the nickel powder is a mixture of particles that have two different morphologies. One of the morphologies of the nickel powder is essentially rounded or spherical and the second morphology is dendritic. The B.E.T. specific area of one of the morphologies of the particles can be in the range of 0.25-0.35 m.sup.2 /gram, whereas the B.E.T. specific areas of the other morphology of particle can be in the range of 0.38-0.50 m.sup.2 /gram. The ratio between the weight of the different forms of nickel morphologies may range from about 4:1 to 1:2.5. Stated another way, the different morphologies of nickel powders may be present in form of a mixture comprising at least 28 % of particles of a first morphology and at least 20% of particles having another morphology.